The present invention is directed to a thyristor device with self-protection against breakover turn-on failure and more specifically to a device which achieves its self-protection through planar junction curvature effects.
The main emitter area of a thyristor is very prone to failure during breakover turn-on initiated by excessive device voltage. The location of the turn-on point (usually the location of the maximum avalanche current) is not subject to control and will usually be situated somewhere under the cathode emitter rather than in some more desired location such as under the gate area of the device.
One prior technique which sought to achieve initial breakdown under the gate area is described in an article in Solid State Electronics, Volume 27, page 655, 1974, by Peter Voss. That article discloses a base region which has been carefully prepared so that its highest donor concentration is located precisely under the gate contact. The dependence of avalanche breakdown on such concentration assures that this area will breakdown first and thus protect the device. Naturally such adjustment of donor concentration is difficult.
Another method involves external circuitry which is connected between the anode and the gate of the thyristor. The breakover voltage of this external circuit is adjusted so that the circuit itself breaksover before the main emitter of the thyristor which is to be protected. Thus the gate of the device is fired in the normal manner. This method is, however, costly and requires extra components.
Two copending applications disclose and claim other possibilities for the local lowering of breakdown voltage in transistor and thyristor structures. These make use of the fact that alpha times M being equal to one is a turn-on criteria where alpha is the current gain or the base transport factor of the structure and M is the avalanche multiplication factor. In one copending application in the names of Temple and Baliga the selected base life time control is used to increase alpha and lower the breakover voltage. In the other copending application, also in the names of Temple and Baliga, Ser. No. 737,385, filed Nov. 1, 1976, pre-etch techniques are used to provide a locally thinner base. Both the above possibilities suffer from the fact that the device will be more sensitive to dV/dt and to thermal leakage current turn-on than if M alone is made a larger valued function of voltage. This makes them somewhat more complicated to apply without loosing some dV/dt capability.